In most blasting operations different charges in a round are triggered sequentially with a certain time delay between individual charges or groups of charges. This makes possible control of the rock movements during blasting, in order, for example, to maintain a free expansion surface for all charges in the round, to affect rock fragmentation and displacement, and to control the ground vibrations.
The delay is achieved conventionally by means of a pyrotechnical delay element arranged in the detonator, the length and burning rate of which element determine the delay time. When the delay element has been fired by the initiation signal, it burns at a predetermined rate and subsequently initiates the explosive in the detonator. A certain time scatter is, however, unavoidable even in the case of accurately produced pyrotechnical elements. Since a relatively large number of different delays are required, delay elements of different pyrotechnical compositions and burning rates must be used, which increases the risks of undesired scatter because of the different ageing properties of the various elements. Moreover, because the pyrotechnical delay element has a given burning time, a large range of detonators must be produced and stocked. For reliable ignition the element must rest against the explosive in the detonator, which makes it difficult, in the field or on the premises, to assemble the desired range of detonators.
Different proposals for electronic detonators have been put forward in which the pyrotechnic delay is replaced by an electronically generated delay. By this means the precision of the detonator delay time can be considerably improved and also made non-sensitive to storage. If the detonator is made programmable, the same detonator type can be used for many different delays, and possible delay times can be chosen at will and do not require to be standardized in advance. Apart from the electronics part, the detonator can be made as simple as a normal instantaneous detonator.
Commercialization of electronic detonators has been held back by several problems. It has been found difficult to reduce the price of the relatively complicated electronic circuit to the level of the pyrotechnic element. Even if the major part of the electronics can be designed as a single semiconductor chip, the circuit solution must in addition comprise at least one discrete component, such as, for example, a current source for powering of the electronics during the delay phase and for ignition of the fuse head. These components and their mutual electrical and mechanical connections increase considerably the costs of the electronic detonator. The circuit must, in spite of the easily damaged components, satisfy essentially the same mechnacial strength requirements as the considerably more robust parts of a pyrotechnic element, i.e. withstand relatively careless handling during assembly of the detonator, during connecting up of the round, and during severe ground vibrations and shock waves from adjacent detonations during the delay phase. A strong mechanical construction does however conflict with the desired objective of being able to produce the electronics detonator in the same shell dimensions as previously, which have been more or less standardized, and of being able to use the existing assembly equipment. Reliable ignition imposes limitations on the possibilities of reducing the size and electrical energy requirement of the fuse head. The precision of the electric delay is counteracted by the dead time and the resulting time spread in the remaining parts of the firing chain, such as the fuse head and charges in the detonator. The possibility of reducing the response time of the fuse head is limited by the capacity of the current source. Miniaturization of the electronics, which is desirable per se, increases the sensitivity to static electricity and other disturbances, which, in the context of explosives technology, represents a safety problem. The mechanically sensitive electronic components also make difficult the final assembly of the detonator and in particular the possibilities of simple local assembly of prefabricated parts.